All-optical Tunable Narrow-band Organic 2007

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DOI: 10.1007/s00340-007-2578-x

Appl. Phys. B 87, 255258 (2007)

Lasers and Optics

Applied Physics B

x.hu

p. jiang

c. ding

q. gong

All-optical tunable narrow-band organicphotonic crystal filters

State Key Laboratory for Mesoscopic Physics, Department of Physics, Peking University,Beijing 100871, P.R. China

Received: 17 October 2006

Published online: 22 February 2007 Springer-Verlag 2007

ABSTRACT An all-optical tunable narrow-band filter with an ul-trafast response time of 10 ps is realized in a two-dimensionalnonlinear polystyrene photonic crystal. The pump and probescheme is adopted to measure tunability based on the picosec-

ond optical Kerr effect. The passband of the photonic crystalfilter shifts about 4 nm under excitation of 14.7 GW/cm2 pumpintensity, which is in agreement with the theoretical prediction.

PACS42.70.Qs; 61.46.+w; 81.15.-z

1 Introduction

Recently, tunable narrow-band photonic crystal fil-ters have attracted great attention due to their important ap-plications in the fields of optical interconnection network andultrahigh speed information processing. Many schemes have

been proposed to construct narrow-band photonic crystal fil-ters, such as the cascaded identical resonant grating [13],the coupled-cavity waveguide [46], the single photoniccrys-tal slab [7] or the photonic crystal waveguide coupled withmicrocavities [8, 9]. Villa et al. demonstrated that surfacemodes in the junction of two different one-dimensional pho-tonic crystals placed in series could also be used to formnarrow-band filters [10]. In 2003, Li et al. reported a 25nmshift for the passband of a photonic bandpass filter in a one-dimensional silicon photonic crystal by modifying the latticeparameters [11]. Recently, Chen et al. achieved a shift in thecenter wavelength of the passband of about 23nmin a one-dimensional GaAs/AlO narrow-band photonic crystal filter

by changing the thickness of the defect layer [12]. However,little attention was paid to the time response of the tunablenarrow-band photonic crystal filter up to now.

The aim of this letter is to achieve an ultrafast time re-sponse for tunable narrow-band photonic crystal filters. Forthis purpose, we adopted polystyrene to fabricate a two-dimensional photonic crystal filter due to its subpicosecondnonlinear response [13]. According to the nonlinear opticalKerr effect, the refractive index of polystyrene varies with thepump intensity, which leads to the changes of the position ofthe passband in the photonic bandgap. An ultrafast response

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time of10ps was achieved for the tunable photonic crystalfilter.

2 Experimental

Polystyrene powder with a normal molecular

weight of8000000(Fluka Chemie Company, Switzerland)was dissolved in toluene with a weight ratio of1 : 140. Thespin-coating method was used to fabricate polystyrene filmswith a thickness of300 nmon silicon dioxide substrates [14].The refractive index of silicon dioxide is smaller than that ofpolystyrene, which ensures excellent control of light in thevertical direction. A focused ion-beam (FIB) etching system(Model DB235, FEI) was employed to prepare the periodi-cal patterns of a photonic crystal filter. The Ga+ ion beamgenerated by a Canion ion gun was connected to an ultrahighvacuum chamber, where the sample was placed. A spot cur-rent of30pAwas obtained from a weak emission current of1A at 25 keV. The fabrication process is detailed in [15].

The sample was composed of four line defects with a widthof310 nmin the center of regular square arrays of cylindricalair holes embedded in the background matrix of a polystyr-

FIGURE 1 Surface view: scanning electron micrograph of the two-dimensional photonic crystal filter


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256 Applied Physics B Lasers and Optics

FIGURE 2 Experimental setup. The thick lines represent optical connec-tions, while thin linesare electronic connections

ene slab. A scanning electron microscopy (SEM) image of thetwo-dimensional narrow-band photonic crystal filter is shownin Fig. 1. The radii of theair holes andthe lattice constant were90nm and 220 nm, respectively. The total etched area wasabout2.5100 m. The highly ordered and periodic struc-tures of air hole patterns indicated the perfect quality of thephotonic crystal filter.

The evanescent-field coupling technique [15] was usedto measure the transmittance spectrum of the photoniccrystal filter. The experimental setup is depicted in Fig. 2.A 1.064m beam (with a pulse duration and pulse repe-

tition rate of 25ps and 10Hz, respectively) from a YAGlaser (Model PL2143B, Ekspla) was used as the pump light,which was normally incident to the upper surface plane ofthe photonic crystal filter. The beam (pulse duration 10psand repetition rate 10Hz) from an optical parameter amplifier(OPA) (Model OPA-740, CAS) pumped by the YAG laser wasused as the probe light. The photonic crystal filter was con-nected with two polystyrene waveguides with a thickness of300 nm. The probe laser was incident onthe bottom ofa prismwith a high refractive index, which was placed above the up-per surface of the polystyrene waveguide. The incident angleof the probe laser was adjusted sothat the condition of total in-ternal reflection was met and the propagation constant of the

probe laser was equal to that of the guided electromagneticmodes of the polystyrene waveguide. As a result, the probelaser was coupled in the polystyrene waveguide with the helpof the evanescent field generated in the air gap between theupper surface of the polystyrene waveguide and the bottomof the prism. The probe laser propagated through the pho-tonic crystal in theXdirection, which was parallel to theline defect. Both the probe and pump laser were TE polarized

waves with their electric-field vectors parallel to the polystyr-ene film. A delay line was used to adjust the temporal relationbetween the pump and probe pulse. The light transmittingthrough the photonic crystal was detected by a monochroma-tor, whose output signals were magnified by a photomultiplierbefore they were input into an oscilloscope. Finally, a com-puter was used to collect and handle the output data from theoscilloscope.

3 Results and discussion

The transmittance spectra of the photonic crystalfilter are depicted in Fig. 3. Due to the limitation of the oper-

ation frequency range of the OPA, the transmittance spectrumof the photonic crystal filter could not be measured whenthe wavelength was lower than 430 nm. The central wave-length and the estimated bandwidth of the passband of thephotonic crystal filter were496 nmand112 nm, respectively.

FIGURE 3 Transmittance spectra of the photonic crystal filter. (a) The

blocksrepresent the measured values. (b) The linerepresents the simulation

FIGURE 4 Changes of the probe light transmittance as functions of the

time delay between pump and probe pulses. The wavelength of the probelight and the pump intensity were 551 nm and 14 .7 GW/cm2, respectively


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HU et al. All-optical tunable narrow-band organic photonic crystal filters 257

FIGURE 5 Tunability of the pho-tonic crystal filter. (a) Shifts of the

long-wavelength edge of the pass-band with different pump intensities.(b) Calculated transmittance spectraof the passband with different pump

intensities by the multiple scatteringmethod

The measured results were in agreement with the theoreticalones calculated by the multiple scattering method [16]. Whena line defect is introduced in a perfect two-dimensional pho-tonic crystal, the periodicity of the spatial distribution of the

dielectric materials is destroyed. This leads to the formationof localized defect modes with high transmittance in the pho-tonic bandgap [17]. Kee et al. pointed out that the couplingbetween two identical localized modes makes their eigenfre-quency split into a lower frequency mode and a higher fre-quency mode [18]. When a number of identical defect unitsare introduced in a photonic crystal, a transmittance band canbe formed [19, 20]. This results in the formation of the widepassband of the photonic crystal filter. The transmittance ofthe passband changes slightly, which indicates that the pass-band possesses a flat top. The average transmittance of thepassband was more than 80% and the transmittance contrastbetween the passband and the stop band was higher than 60%.

The high transmittance and the steep roll-off of the passbandimply that the photonic crystal filter possesses excellent filter-ing properties.

In order to determine the time responseof the tunable pho-tonic crystal filter, we measured the transmittance changesof the probe light as functions of the time delay between thepump and probe pulse. The measured results are shown inFig. 4. The wavelength of the probe light and the pump inten-sity were551 nmand 14.7 GW/cm2, respectively. It is veryclear that the transmittance changed only when the pump andprobe pulse overlapped with each other. The maximal trans-mittance was obtained for a zero time delay, with two pulsesoverlapping completely in the temporal domain. Moreover,

the half width of the signal envelope, 10ps, was in proxim-ity to the pulse duration of the pump light. The signal profileshowed an almost symmetrical distribution around the zerotime delay. This evidence showedthat thetime responseof thetunable photonic crystal filter was faster than the experimen-tal time resolution [21]. So, the measured time response of thetunable photonic crystal filter, 10ps, is limited by the pulseduration of the pump light.

To study the tunability of the photonic crystal filter, thetransmittance changes of the551 nmprobe light as functionsof pump intensity were measured. The shift of the long-wavelength edge of the passband with different pump intensi-ties is depicted Fig. 5a. The shift magnitude of the passband

increases with the increment of the pump intensity. Accord-ing to the nonlinear Kerr effect, the positive value of thethird-order nonlinear susceptibility of polystyrene results inthe increase of the effective refractive index of the photonic

crystal under the excitation of the pump light, which makesthe passband of the photonic crystal filter shift in the long-wavelength direction. The maximal shift was 4.2 nm under14.7 GW/cm2 pump intensity, which was in agreement withthe calculated results. The transmittance spectra of the pass-band with varying pump intensity calculated by the multiplescattering method is shown in Fig. 5b. The average transmit-tance and the bandwidth of the passband changed slightlyunder the excitation of the pump light, which shows that thephotonic crystal filter possesses very excellent tunability.

4 Conclusion

In conclusion, we have realized a tunable narrow-band photonic crystal filter with an ultrafast response time of10ps. These results may be valuable references for the studyof integrated photonic devices with ultrafast time response.

ACKNOWLEDGEMENTS This work was supported by the

National Natural Science Foundation of China under grants 10574007,

10521002, 10434020, 10328407, 60378012, and 90501007, and the Na-

tional Basic Research Program of China under grants 2007CB307001 and

2006CB806007.

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All-optical Tunable Narrow-band Organic 2007